The field of art to which this invention pertains is aryl cyanate esters, i.e., cyanic acid esters of polyhydric phenols.
Industry is constantly searching for lighter, stronger and more resistant materials to be used in place of the materials used today. For example, the aerospace industry is devoting considerable effort to utilizing structural composites in place of metals. Structural composites based on thermoplastic or thermoset resins and glass or carbon fibers have been and are being used successfully in many parts of military and commercial aircraft. Thermoset resins which are being used in such applications are epoxy resins, bismaleimide resins, and cyanate ester resins.
Cyanate ester resins, which are finding increasing use in structural composites, adhesives and electrical grade insulation are based on the reaction products of polyhydric phenols and cyanogen halides. Such resins and their methods of preparation are described in U.S. Pat. Nos. 3,403,128 and 3,755,042. Additional patents which describe cyanate esters are U.S. Pat. No. 3,987,230 and 4,330,658.
Such cyanate esters are generally crystalline in form but can be heated to form amorphous prepolymers which are partially trimerized resin intermediates. However, such homoprepolymers have a tendency to partially crystallize with time. Crystallized materials are difficult to handle in commercial operations and require extra heating to convert them to the amorphous form for ease of handling. Non-crystallizing homoprepolymers formed by increasing the degree of trimerization to 30 percent or greater have viscosities somewhat higher than prepreg manufacturers and fabricators of filament wound composites would like to use.